And those Kardashian women — has each had the bag surgically connected to her arm? The bag seems as vital to them as air and flashbulbs.

The bag is Hermès’ Birkin Sellier 40, it costs $15,000 and up — and there’s a waiting list of a year or longer to buy one.

If you can even get on to the waiting list.

“Hermès has created a culture in which there are wait lists and top clients. You may have to wait a couple of years to buy Birkins and Kellys, the really coveted bags,” said Caitlin Donovan, an accessories specialist at Christie’s New York.

Kelly is believed to be the second-most coveted Hermès bag in the company’s lineup.

Model Irina Shayk sports her Birkin bag around NYC.Splash News

“There are waiting lists and politics involved. Clients build relationships with sales people to qualify to buy a couple of bags per year. You are limited to a certain number.”

What makes the bag perhaps the most coveted carryall in the world is difficult to pin down, but its price, scarcity and craftsmanship would certainly lead the list.

A visit to Hermès’ Midtown Manhattan store last week found just one Birkin bag on display — a $65,000 model, in a four-sided glass display case in the middle of the store.

A saleswoman told a shopper that wait times to get one “depend on your relationship with the store” and that customers must first complete a profile to become a client.

This exclusivity has given birth to a booming secondary market in Birkin and Kelly bags.

Resellers like Trendlee.com, Poshmark.com, LuxeDH.com and Heritage Auction are now offering deals on even the most exclusive Birkin and Kelly bags — and Christie’s, the auction house, sells the models in its recently launched online handbag store.

The demand is so strong that at LuxeDH, a new $17,000 Birkin sold for $25,000, according to Roberto Szerer, the site’s chief executive.

“The buyer bought it at the higher price because she simply wanted it and didn’t want to wait,” said Szerer.

Don’t have $25,000? No problem. LuxeDH offers payment plans.

The resellers get their bags from collectors and everyday luxury handbag fanatics who sell the bags to the sites — often flipping a Birkin for a quick profit.

One Hermès bag owner sold a brand-new custom-made Birkin at a Christie’s auction in May for $95,000.

Online auctioneer Paddle8, which just raised $34 million, began hawking pre-owned luxury bags this year and will soon host monthly auctions.

The competition doesn’t seem to be hurting Hermès — yet.

In the first half of 2015, Hermès operating profit and revenue each grew by about 20 percent — and its leather goods was the best performing unit.

But the French luxury-goods maker can’t be too happy to see the resellers jumping into the market.

“The brands want customers to buy the bags from their boutiques,” said Gina Nixon, founder of 2-year-old Authenticate First, which is hired by resellers to ensure that bags are not fakes.

One of Nixon’s regular clients flips bags she buys on eBay in Japan, where upscale consumers shed older luxury bags to get the latest model — as if they were iPhones.